Question

1) Describe (absolutely no pictures) the life cycle of a dsRNA virus.

2) Describe a Medical Difference or Therapeutic Target in the life cycle of an RNA virus.
types which a particular virus is able to infect.” Describe how the process of adsorption is related to
the host range of a virus.

Answer 1

(1). ds RNA stands for double stranded RNA. It is found in host cell cytoplasm . The transcription of ds RNA occurs in the mRNA . Transcription, defined as the synthesis of viral (+)-strands from a dsRNA template, takes place within viral particles or core particles. One interesting exception is the dsRNA replicons that mitigate the virulence of the chestnut blight fungus, Cryphonectria parasitic. These are found in membranous vesicles and lack a substantial protein coat .

Following are the steps of replication :-

A. Attachment. This is the first step in viral replication. Surface proteins of the virus interact with specific receptors on the target cell . surface. These may be specialized proteins with limited distribution or molecules that are more widely distributed on tissues throughout the
body. The presence of a virus-specific receptor is necessary but not sufficient for viruses to infect cells and complete the replicative cycle.

B. Penetration. Enveloped viruses (e.g., HIV, influenza virus) penetrate cells through fusion of the viral envelope with the host cell membrane. Non-enveloped viruses penetrate cells by translocation of the virion across the host cell membrane or receptor mediated endocytosis of
the virion in clathrin coated pits with accumulation of viruses in cytoplasmic vesicles.
C. Uncoating (disassembly). A complex process which differs by taxonomic class and is not fully understood for many agents. This process makes the nucleic acid available fortranscription to permit multiplication of the virus.
D. Transcription and Translation. The key to understand the genomic expression of viruses is that viruses must use host cellular machinery to replicate and make functional and structural proteins.

E. Assembly and Release. The process of virion assembly involves bringing together newly formed viral nucleic acid and the structural
proteins to form the nucleocapsid of the virus. There are basically three strategies that viruses employ:
1. Nonenveloped viruses exhibit full maturation in the cytoplasm (e.g., picornaviruses) or the nucleus (e.g., adenoviruses) with disintegration of the cell and release of virions.

2. For enveloped viruses, including the (-) strand RNA viruses, the (+) strand togaviruses and the retroviruses, final maturation of the virion takes place as the virion exits the cell. Viral proteins are inserted into the host cell membrane. Nucleocapsids bind to the regions of the host cell membranes with these inserted proteins and bud into the extracellular space. Further
cleavage and maturation of proteins may occur after viral extrusion to impart full infectivity on the virion. Viruses in this group differ in their degree of cytolytic activity.

3. Herpesviruses, which are enveloped viruses, assemble their nucleocapsids in the nuclei of infected cells and mature at the inner lamella of the nuclear membrane. Virions accumulate in
this region, in the endoplasmic reticulum and in vesicles protected from the cytoplasm. Release of virions from the cell surface is associated with cytolysis.

(2)

MicroRNAs (miRNAs) are endogenous RNAs that can play important regulatory roles in animals and plants by targeting mRNAs for cleavage or translational repression. miRNAs and endogenous siRNAs have a shared central biogenesis and can perform interchangeable biochemical functions. Therefore, the two classes of silencing RNAs cannot be distinguished by either their chemical composition or mechanism of action . These short RNA species are produced by Dicer cleavage of long endogenous precursors with imperfect hairpin RNA structure in animals. Mature miRNAs act as a translational repressor by partial base-pairing with 5′ or 3′ ends of mRNAs while miRNA completely complementary to its target mRNA (endogenous siRNA) can result in the degra-dation of cognate mRNA.

Adsorption:

In most cases, specific attachment proteins on the surface of viruses bind specific receptors on the surface of animal cells. Cellular receptors are usually either glycoproteins or glycolipids, and have other functions for the cell in addition to virus binding. The specific interaction between attachment proteins and cellular receptors is a major determinant of the host-range, or tropism of the virus. Some viruses have a very narrow host range, meaning that they can only infect one or a small number of cell types, while others have broad host ranges, meaning that they can infect a large number of different cell types. This is partially determined by whether the receptor for the virus is expressed on many or a limited number of cell types.